This invention pertains to reducing the effect of noise in video systems. The principles of the improved noise reduction means will be illustrated herein primarily as they are implemented in digital x-ray fluoroscopic video display systems.
In the system in which the improved noise reduction means is exemplified the beam from an x-ray source is projected through an object such as a body under examination and the image produced thereby is intercepted by an image intensifier. The image intensifier converts the x-ray image to an optical image. The optical image is received by a television camera which converts it to the usual video signal waveforms on a line-by-line basis. The amplitude of the video signal derived from an x-ray image varies linearly with brightness of the x-ray image. Hence, in order to obtain a linear relationship between signals corresponding to image brightness and object thickness, the video signals must be amplified logarithmically. After logarithmic amplification, the analog video signals are converted to digital data. There is a digital number or value for each picture element (pixel) derived from the analog video signal. The digital data are processed, stored in a full image memory and ultimately used to drive a television monitor that enables the radiologist to view an optical replica of the x-ray image.
As is well known, it is desirable to take measures to improve the signal-to-noise ratio in ordinary monochrome and color television systems and in x-ray fluorographic television display systems as well.
Several schemes have been proposed for noise reduction or signal-to-noise ratio improvement in monochrome and color television systems. A noise reduction system for commercial television systems that is believed to be particularly effective is described in U.S. Pat. No. 4,064,530. However, the patented system does not fulfill some of the requirements of analog and digital x-ray fluorographic television display systems. In medical fluoroscopy, where logarithmic amplification is used, the noise level changes with the useful video signal level. In the logarithmically amplified signal, the noise is generally greater in the dark portions of the image and smaller in the whiter portions. Thus, it has been found to be desirable to provide for greater noise reduction in the darker than in the whiter regions, but, insofar as is known, this has not been achieved heretofore.
The method described in the cited patent is based on the fact that the useful video signals occur substantially periodically whereas noise occurs randomly. The system uses a full image memory or digital frame store for the digitized pixels making up the successive television frames. For fluorographic purposes, there might be, typically, 512 rows of pixels in each line and 512 lines comprising a television frame. The frame store or memory acquires and the system processes video images at the standard rate of 30 frames per second so the image can be displayed on a television monitor at standard television rates.
The noise reduction system in the cited patent is based on averaging respective digitized pixel signals for many television frames on a continuous basis and reading out the average pixel values from the frame store for conversion to analog equivalents and for display by the monitor thereafter. Because averaging tends to cancel noise, the averaged image appears less noisy on the monitor display screen.
In the cited patent, averaging is done recursively. In general, a controllable fraction of each digitized stored pixel is added to a controllable fraction of the digitized or real-time live video image coming from the video camera or a tape recorder. The result is restored to the frame store memory pixel locations and is repeated at the video frame rate every 1/30 of a second. The prior system detects motion in individual pixels by detecting the difference between the magnitude of the stored and live images on a pixel-by-pixel basis. For various differences below a threshold value, the fraction added varies with the value of the difference. If the detected difference exceeds threshold, the averaging process is turned off for that pixel and the live pixel merely replaces the stored pixel value as output from the digital frame store. If the live pixel were not sent through unmodified when there is motion, the moving edges in the displayed image would have long trailers behind them, thus exhibiting a smeared image.
Since in the patented system the fraction of the live and stored pixel value used depends on the difference between the live and stored or averaged pixel values, when there is substantial motion of an object in an image scene relative to background, there would be a great difference between the live video signal and the stored averaged pixel values along the edge of the moving object so noise reduction is decreased for the pixels in motion. In the extreme case noise reduction by averaging is completely eliminated so the live pixel corresponding to the edge of the object itself is displayed rather than some modified difference between the live and stored video signals.
There is a reason for discontinuing averaging and letting the live video signal come through when there is motion. The reason can be appreciated if one recognizes that the averaged pixels are stored in a format such as a 512.times.512 format, by way of example. If the image has been static for a period of time the valid or useful video signal and the noise components will be averaged and be represented by the existing digital value of the pixels accumulated during a succession of television frames. Now consider any one of the pixels at the edge of an object which can move in the scene. Since the live and stored or averaged pixels that correspond are subtracted on a pixel-by-pixel basis, the single pixel under observation at the edge of the object which coincides with a corresponding averaged value during one frame will be in coincidence with a different averaged value during the next successive frame. Thus, the moving pixel which truly corresponds with the edge of the moving object would have successively different averaged values subtracted from it and smearing of the displayed image would result. Hence, live pixels are allowed to come through unmodified or without noise reduction at the edges of any object whose motion has been detected.